西沙地区碳酸盐台地发育过程与海平面变化: 基于西科1井BIT 指标分析数据
Carbonate Platform Development and Sea-Level Variations of Xisha Islands: Based on BIT Index of Well Xike-1

为了探讨南海碳酸盐台地的发育过程及控制因素,采用有机分子化合物指标方法对其进行了研究.有机分子化合物指 标BIT(branchedisoprenoidtetraether)是沉积物中源自陆源的细菌膜脂支链甘油双烷基链甘油四醚(branchedglyceroldialkylglyceroltetraethers, 简称bGDGTs)与主要来自海洋泉古菌中的类异戊二烯GDGTs(isoprenoidGDGTs,简称iGDGTs)的 含量之比,在古环境研究中,用来区分沉积物有机质的来源、判断沉积环境.通过对西科1井的数据研究发现,西科1井BIT 指数随深度呈现三段式规律性变化,从下到上呈现高-低-高的变化,反映了中中新世以来南海海平面变化及碳酸盐台地生 长发育的过程:在中中新世晚期,受全球及区域海平面变化下降影响,西沙地区碳酸盐台地形成礁-滩交互的沉积地层,由于 大气淡水的影响,造成BIT 指数呈现高值;到晚中新世至上新世,全球及区域海平面出现持续上升,有利于西沙碳酸盐台地的 生长发育,使该井沉积环境以礁内泻湖相为主,造成BIT 指数呈现低值;在第四纪冰期,全球及区域海平面出现总体下降趋 势,西沙碳酸盐台地又频繁暴露于地表,造成BIT 指数又呈现高值.研究表明,西沙地区碳酸盐台地受区域相对海平面变化影 响更大,说明南海海平面变化既受全球海平面变化的影响,也受南海区域构造沉降的控制.
In ordcr to cxp1orc thc dcvc1opmcnta1 proccss of carbonatc platform in thc South China Sca(SCS) and its contro1 fac- tors, the organic molecular compound index method were described in this study. The branched isoprenoid tetraether(Bl「T) in- dex is the ratio between the branched glycero1 dialky1 glycero1 tetraethers(bGDGTs) from terrigenous supply and isoprenoid GDGTs(iGDGTs) from marine crenarchaeo1 comprising the majority of denominator, which serves as a proxy for provenance discrimination of the organic matter and related sedimentary environments in paleoenvironmenta1 research. The B.1'T curve of we11 Xike-1 is characterized by a unique“high-1ow-high” feature up section, indicating the effects of sea-1eve1 variations and car- bonate plat「orm deve1opment since Middle Miocene in the SCS. Strata with reei-beach iacies were deve1oped within carbonate platforms of Xisha Islands influenced by decrease in g1oba1 as we11 as regiona1 sea-1eve1 during the Iate Miocene, with increased 131lT values due to plenty of precipitation. Between Late Miocene and Pliocene when sea-1eve1 was higher, carbonate platforms were better formed in sha11ow marine environment with1ow Bl「T values. Carbonate platforms of Xisha Islands were periodica11y exposed in Quaternary as a result of sea-1eve1 decline over glacia1 cycles, leading to the second increase in BlfT. 〇ur results demonstrate that carbonate platforms of Xisha Islands have been affected by regiona1 sea-1eve1 variations, reflecting a coupling effect on the SCS by g1oba1 sea-1eve1 fluctuations and regiona1 tectonic subductions